1. Field of the Invention
This invention relates to high brightness Light Emitting Diodes (LEDs) and methods and devices that improve light extraction efficiency of LEDs without substantially increasing device cost.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
There are several previous publications dealing with the use of Zinc Oxide (ZnO) nanorods for enhancing light extraction efficiency from GaN LEDs. Zhong et al. report on a metal organic chemical vapor deposition (MOCVD) grown ZnO nanorod array, on top of a ZnO:Ga transparent contact layer deposited on (0001) p-GaN [1]. Compared to a conventional Ni/Au contact, they report a ˜1.7 times increase in light emission efficiency.
Also using MOCVD (or MOVPE) growth, An et al. then reported on the light output from the same device, before and after growing ZnO nanorod arrays on the surface [2]. Here, the nanorods reportedly produced a ˜50% increase in output at 20 mA, and a ˜100% increase at 50 mA. Despite these improvements in light extraction, these methods are unlikely to see commercial application due to the high cost and low throughput that would be associated with using MOCVD growth for of ZnO nanorods.
However, Kim et al. recently reported on the growth of a ZnO nanorod array using an aqueous solution route [3]. In this case, the nanorods were able to improve light output by almost 60%. Compared with MOCVD growth, aqueous solution deposition can be a lower cost and higher throughput method [4]. This could drastically reduce the cost and make ZnO nanorod arrays a commercially feasible option for enhancing the light extraction efficiency of GaN LEDs.
All of the reports described above dealt solely with ZnO nanorod arrays situated on the c+-plane (0001) surface of the LED, either deposited directly onto the p-GaN or onto an intermediate layer. However, according to the present invention this is not the only type of LED device surface where ZnO nanorods could potentially enhance light extraction. For instance, in most laser lift off device configurations nanorods would instead need to be grown on the c− plane (000-1).
For devices grown on bulk GaN substrates, the present invention describes that ZnO nanorod arrays could potentially improve light extraction from any of the surfaces of the device. Depending on the LED's orientation and cut, this could include the polar c+ and c−-planes, the non-polar m- and a-planes, and numerous semi-polar planes. In some cases it may also be useful to produce nanorods on top of non-GaN LED device surfaces, such as substrates, and transparent contact or current spreading layers.